Field of the Invention
The present invention relates to heating assemblies and more particularly to fuel burners for furnace heating assemblies and a method of making the same.
Heating assemblies for metallurgical or heat treating furnaces raise the furnace atmosphere to a set point temperature range rapidly and then maintain the furnace atmosphere in the set point temperature range for a desirable period of time. The heated furnace atmosphere is circulated between a number of the heating assemblies and a furnace work zone containing material to be heat treated. The heating assemblies are commonly formed by a heating tube and a fuel burner disposed within the tube for maintaining a tube wall heating flame. The tube wall, in turn, transfers heat to the furnace by radiation and by convective heat transfer. The heating tubes exhaust outside of the furnace and thus serve to isolate the furnace atmosphere from the gasses in the tubes.
The prior art burners have generally been supplied from a source of fuel (e.g. natural gas) and combustion air and have introduced mixed fuel and air into their heating tubes. The fuel and air should be mixed in proportions to assure optimum combustion, i.e., a so-called stoichiometric fuel-air mixture ratio should be provided. Combustion has been initiated by a pilot flame associated with each heating tube remote from the burner and, once established, combustion occurs in a flame which extends from within a discharge section of the burner through the heating tube to a location where the fuel is consumed.
The mass flow rate of the fuel and air supplied to the burner is variable and governed thermostatically according to sensed furnace atmosphere temperature to maintain the furnace atmosphere in a desired set point temperature range. The furnace is initially heated into the set point range by combustion of the mixture flowing through the burners at a maximum flow rate. This is referred to as the "high fire" condition in that a large flame is maintained in the heating tube. When the set point temperature is reached, the mixture flow is throttled back to a predetermined level where the furnace atmosphere temperature remains constant, or decreases gradually. This condition is referred to as the "low fire" condition. If the temperature drops below the set point range the maximum fuel-air flow rate is reestablished to rapidly increase the furnace atmosphere temperature.
The burners tend to become extremely hot during furnace operation because of their direct exposure to the flame. The heat, coupled with the oxygen rich atmosphere ambient the burner, is quite destructive and the burners are eventually consumed, albeit gradually, beginning with the material forming the burner discharge opening. Burners have conventionally been constructed from so called high temperature alloys which resist consumption to maximize the effective burner life and reduce the costs attendant to burner replacement. High temperature alloys which are most frequently used for burners have exhibited extremely poor machinability resulting in the burners being rough in that they are characterized by having generally poor surface characteristics, burrs, etc. The rough nature of the burners tends to create erratic fluid flows which accelerate their consumption rates and the consumption occurs unevenly. These factors result in unstable, poorly formed flames in the heating tubes, particularly under "low fire" conditions.
Unstable and/or poorly formed flames are undesirable because they reduce the overall furnace efficiency. These flames often produce zones of maximum combustion heat in the central portions of the heating tubes remote from the walls. This results in furnace inefficiency due to failure to maximize the transfer of heat to the tube walls from the flame.
Unstable flames also tend to promote explosive back flashing of unburned fuel in the heating tubes when the burner condition is altered from "high fire" to "low fire", as well as in those circumstances where the burner flame is lit and relit during the furnace operation. When the fuel flow rate is reduced to the low fire level the mixture flow configuration is believed to change significantly in circumstances where unstable flames are present. This can result in movement of the flame base away from the burner tip while unburned fuel and air continues to be introduced into the tube behind the flame base. The unburned mixture accumulates until lit by the heating tube pilot flame remote from the burner. The resultant back flash tends to be sufficiently explosive that the heating tube wall can be broken particularly when the fuel-air mixture is not properly adjusted, which is not an infrequent occurrence.
Still further, unstable flames often create heating tube wall hot spots. The existance of a tube wall hot spot creates localized wall temperature differentials and consequent localized tube wall stresses. These stresses can be sufficiently great to crack the tubes.
Furnace heating tubes are fragile and when broken are expensive and difficult to replace. The heating tubes have generally had thin ceramic walls suitable for radiating heat efficiently and capable of withstanding the effects of extremely high temperatures, chemically active environments and rapid temperature changes without loss of structural integrity. These materials are brittle, have relatively low tensile strengths and if a tube breaks or cracks the furnace atmosphere can be contaminated by products of combustion. Accordingly, broken tubes must ordinarily be replaced promptly to avoid possible damage to furnace contents.
Breakage and replacement of furnace heating tubes has been a significant problem. The heating tubes are relatively expensive in themselves and the costs attributable to labor and furnace downtime required for tube replacement are high. Heating tubes have been broken rather frequently by stresses resulting from unequal heating of the tube walls and explosive back-flashing of unburned fuel in the tube. For the reasons noted these causes are attributable to the construction of the fuel burners.
One widely used prior art burner is formed by a one piece cylindrical tube of heat resistant metal having an inlet section, a reduced diameter throat section and a discharge section. The inlet section is threaded so that the burner can be screwed onto a fuel supply pipe. Both the inlet and discharge sections are of relatively great length compared to the throat section.
A conical converging wall leads into the throat from the inlet section and series of passages extends through the burner wall parallel to the throat from the inlet section to the discharge section. These passages are spaced circumferentially apart and diverge proceeding toward the discharge section. Each passage is formed by a straight drilled hole having its central axis disposed in a plane coextending with the burner axis. The divergent passages direct the fuel and air mixture onto the discharge section wall. The mixture flowing through the passages tends to be reflected from the discharge section wall back into the main stream of the mixture flowing through the discharge section from the throat section. This construction has produced a turbulent mixture flow and flame, but flame contact with the heating tube walls has not been maximized.
The prior art burner construction referred to has been formed by machining a single piece of relatively heat resistant metal bar stock. Because these alloys generally exhibit extremely poor machinability the burners have been expensive to manufacture, have had relatively short effective lives due to consumption at high temperatures, have contributed significantly to burner tube breakage because of their tendencies to produce unstable and/or poorly formed flames, and have contributed to furnace inefficiency by transferring less than optimum quantities of heat to the heating tubes.
These burners have tended to be consumed by a combination of oxidation and incipient melting which has begun at the tips of the burners and along axially extending regions traversed by the flows from the diverging passages. Consequently the burner discharge passages tend to become shorter and divergent as the burners are consumed. As consumption progresses the burner flames become progressively more unstable which encourages heating tube cracking. When a burner tip is consumed to the point where the diverging passages direct their flows onto the heating tube walls, the resultant tube wall hot spots cause heating tube breakage rather promptly if the burner is not replaced.
The have been proposals to rebuild partially consumed burners of the type referred to by removing the original, partly consumed tips, and replacing them with sleeves which are welded to the remainder of the burner. These proposals were not successful because the weld joints failed rapidly as a result of thermal fatigue and corrosion.